1. Field of the Invention
This invention relates generally to the casting of metal and more particularly, this invention relates to the feeding of molten metal onto a flat planar area of a continuously moving casting surface such as a belt.
2. Background Information
The metals industry has been developing processes and apparatus for producing an as-cast product that needs little or no additional processing such as hot rolling to reduce it to strip form. One such process which has arisen as a result of this development effort is the single belt casting process. According to this process, molten metal is caused to flow onto a moving horizontal surface in the form of a continuous belt whereupon it solidifies as it moves along with the belt. The resulting elongated solid strip of metal is removed from the continuous belt for further processing as desired.
Another process that is undergoing development in connection with the casting of strip products is the use of the spray deposition technique of the type basically developed by Osprey Metals Ltd. of West Glamorgan, United Kingdom. The Osprey process, as it is generally known, is disclosed in detail in U.K. Pat. Nos. 1,379,261 and 1,472,939 and U.S. Pat. Nos. 3,826,301 and 3,909,921 and in publications entitled "The Osprey Preform Process" by R. W. Evans et al, Powder Metallurgy, Vol. 28, No. 1 (1985), pages 13-20 and "The Osprey Process for the Production of Spray-Deposited Roll, Disc, Tube and Billet Preforms" by A. G. Leatham et al, Modern Developments in Powder Metallurgy, Vols. 15-17 (1985), pages 157-73.
The Osprey process is essentially a rapid solidification technique for the direct conversion of liquid metal into shaped preforms by means of an integrated gas atomizing/spray-depositing operation. In the Osprey process, a controlled stream of molten metal is poured into a gas-atomizing device where it is impacted by high-velocity jets of gas, usually nitrogen or argon. The resulting spray of metal particles is directed onto a "collector" where the hot particles re-coalesce to form a highly dense preform. The collector is fixed to a mechanism which is programmed to perform a sequence of movements within the spray, so that the desired preform shape can be generated. The preform can then be further processed, normally by hot-working, to form a semi-finished or finished product.
The Osprey process has been proposed for producing strip or plate or spray-coated strip or plate, as discussed in U.S. Pat. Nos. 3,775,156 and 4,779,801 and European Pat. Appln. No. 225,080. For producing these products, a substrate or collector, such as a flat substrate or an endless belt, is moved continuously through the spray to receive a deposit of uniform thickness across its width.
In connection with the conventional belt casting described above wherein the molten metal is poured onto the continuously moving belt, problems have arisen as attempts have been made to increase the speed of casting (as measured in inches per minute of the cast strip), as well as in reducing the thickness of the cast alloys to eliminate further processing operations. Such problems involve splashing and turbulence arising in connection with the feeding of the molten metal onto the belt which is caused by the momentum transfer between the melt and the belt. The belt must be maintained in relatively cool condition in order to extract heat from the melt to cause the solidification thereof. Accordingly, the source of the molten metal is usually spaced at some vertical distance from the belt to prevent it from heating the belt. When the molten metal is caused to flow directly from the source of the molten metal through the vertical distance onto the belt, the melt will tend to splash when it hits the belt which may result in porosity in the cast Product as well as induce turbulence which can cause inclusions such as oxides. Additionally, there is the problem of achieving the spreading or distribution of the melt evenly across the width of the belt which leads to strip with non-uniform cross-section.
A typical approach undertaken in attempt to solve these problems in belt casting involves the use of a feeding means such as a tundish between the source of molten metal and the belt. Such feeding means normally takes the stream of molten metal issuing from the source and discharges it outwardly in the direction of movement of the belt. Although this approach solves the problem of splashing, the problems of proper spreading of the melt and dampening of its flow to reduce turbulence still remains.
The following references contain a discussion of various delivery systems used for the delivery of the melt to belt casters.
J. Herbertson, P. C. Campbell, A. G. Hunt and J. Freeman, "Strip Casting Studies at BHP Central Research Laboratories", CCC'90 Fifth International Casting Conference, Voest Alpine, Industrieanlangenbau, Linz, June, 1990; J. Herbertson and R. I. L. Guthrie, "A Novel Concept of Metal Delivery to Thin Strip Casters", Casting of Near Net Shaped Products, TMS-AIME, pp. 335-349; and J. S. Truelove, T. A. Gray, P. C. Campbell and J. Herbertson, "Fluid Dynamics in High-Speed Strip-Casting Metal Delivery System", International Conference on New Smelting Reduction of Near Net Shape Casting Technologies for Steel, SRNC-9, J. S. Truelove, pp. 1/10-11/10.
In the case of the spray cast process, according to the basic Osprey process, problems have been encountered due to excessive porosity being present in the cast product. This phenomenon, normally undesirable, is a particular problem in a thin gauge product such as strip since the porous region may comprise a significant percentage of the product. The porosity is thought to occur when the initial semi-solid deposit layer contains too much solid or is cooled too rapidly and provides insufficient liquid to feed inherent intersticies between splatted droplets. Porosity is also thought to occur due to entrainment or entrapment of the atomizing gas.